The Micro-Electro-Mechanical Systems (MEMS) mirrors and mirror arrays have wide applications of light modulation such as optical switches, optical attenuators and optical tunable filter etc. in fiber optic networks. The high filling factor MEMS mirror arrays have particular importance in the wavelength division multiplexing systems. For example they can be used as Optical Cross-connection (OXC) switches, and Wavelength Selective Switches (WSS) etc. The filling factor is generally defined as the ratio of the active/reflective mirror area to the total area of an array. The high filling factor improves optical channel shape and reduces the optical loss in the system. In particular, a micromirror with two dimensional rotations can provide switching of the optical light beam among the channels while avoiding undesirable optical transient cross-talk during switching, and also provides variable optical attenuations.
The MEMS (Micro-Electro-Mechanical Systems) mirrors and mirror arrays also have wide applications in novel optical display, Optical scanners, optical spectrometer, mask less lithography, laser marking and novel light modulation etc.
The critical requirements for the micromirrors and micromirror arrays are larger mirror tilting/rotation angle, high filing factor and lower actuation voltage etc. The larger mirror arrays are required by the telecom industry for optical switches of higher port counts etc.
U.S. Pat. Nos. 7,734,127B2, 7,386,201B1, 6,881,649B2 and 6,822,776B2 disclosed the micromirror and micromirror arrays actuated using electrostatic vertical comb drive actuators. Electrostatic actuation is favored due to its low power consumption and relative simple structure and small footprint.
There are two fundamental obstacles in the above mentioned prior art in order to achieve the larger and high filling factor micromirror array with higher rotation angle and lower actuation voltage. The first obstacle is the extreme difficulty to make the powerful vertical comb drive actuator based on the materials and fabrication methods they chosen. The prior art uses a composite material, silicon/silicon oxide/silicon as the material to fabricate the mirror structure, upper comb fingers and lower comb fingers as well as electrical interconnection wires. In order to have larger actuation capacity or higher energy density of the vertical comb drive actuator, the air gaps between the adjacent comb fingers have to be as small as possible. Also in order to have the larger mirror rotation angle, the height of the upper and lower comb fingers have to be as tall as possible. Narrow air gaps between adjacent fingers and tall comb finger heights result in the high aspect ratio geometry between the adjacent comb fingers during the fabrication of the vertical comb drive. This high aspect ratio geometry will make the etching of silicon oxide (a very inert material to typical plasma chemical etching) between the fingers extremely difficult or sometimes impossible, especially when thicker silicon oxide layer is typically required for a good electrical isolation between upper and lower comb fingers in order to achieve the larger rotation mirror angle at higher actuation voltage.
The second obstacle is the routing of the electrical interconnection wires cross the mirror array chip surface to the bonding pads around the chip edges. Each mirror has five electrical actuation electrodes; one is for the electrical ground, two for +/−X rotation and two for +/−Y rotation. In the mirror array structure, the prior art uses the same silicon-silicon oxide-silicon material to make the electrical interconnection wires, mirror structures and vertical comb drive actuators. These electrical wire structures have to be placed in the spaces between the mirrors. When the mirror array is made larger, the number of electrical interconnection wires increases dramatically. It becomes increasingly difficult to accommodate the large amount of electrical wire structures on the space between mirrors, especially for these mirrors close to the chip edge. Therefore, it is difficult for the prior art to achieve larger and higher filling factor mirror array.